Techshooter's Shooting Pages
by Christopher Long 2004
Shock Wave Theory – Rifle Internal Ballistics, Longitudinal Shock Waves, and Shot Dispersion
Introduction
I started looking at the causes of shot to shot dispersion after getting serious for the first time with loading for accuracy. I ran across Dan’s site (green788) and the concept of OCW. I was intrigued by the undeniable fact that a single load recipe can work so well across many different rifles, with different barrel lengths, diameters, and bedding methods. I started reading everything I could get my hands on regarding barrel vibration and the internal ballistics of a rifle during the firing event. The singular thing that I could not get past was that a simple harmonic vibration pattern could not explain the fact that a single load could work with so many different rifles.
I am a radio communications electronics engineer by profession, and deal with resonance and vibration all the time with antennas and other circuits. It is physically impossible for all the different rifles to have exactly the same harmonic pattern with relation to the bullet exit time. Even a simple change in a barrel contour for a given length will change the resonance, much less a change from a 24†tactical barrel to a 27†target barrel. I am compulsive in the fact that I HAVE to have a clear model and theory of why something works before being comfortable with it. I do bow to expediency often, and just use a process or a machine without questioning how it works, but for anything that I am trying to understand and improve on, I have to have a good model.
Observations and the Resultant Questions
So, I started thinking about other possible reasons for the dispersion of shots within a string. During literally thousands of very careful load charge and seating depth experiments, some notable things were observed:
The point of impact (POI) moved slowly around as the load was increased, the basic premise of Dan’s OCW method.
The size of the group or the dispersion around that POI varied VERY quickly as the seating depth was changed.
The velocity deviation of a load would varied VERY quickly as the seating depth was changed.
Optimum loads work across multiple rifles, with different length barrels.
Even 0.3 grains difference in a 25 grain .223 AI load was enough to take it from a 0.5 MOA group to a 1.2 MOA group. Keep in mind these were not single three shot groups, but typically 2 or more groups of 5 shots each at each load condition to validate the measurement. When I looked a keeping the load constant and changing the seating depth, I saw the same very quick changes in dispersion. As little as 0.010†sometimes affected the groups by the same amount as previously described. The velocity deviation changed rapidly as well. I often observed a reverse correlation between velocity deviation and groups size during these experiments, where loads producing good groups had a high standard deviation of velocity, and vice versa. This was not always the case, in fact, once a truly OCW load was achieved, it became quite tolerant to these changes with respect to dispersion and velocity deviation, again validating Dan’s premise.
Here is what drove me nuts for weeks: A charge change of 0.3 grains or a seating depth change of 0.010†changes the velocity very, very little, usually less than 50 FPS at a mean velocity of around 2900 FPS. The change to the groups, on the other hand, was dramatic.
Why? ? ? -----
----- Summary and Conclusions
So, we now have a theory and a model that can predict the optimum barrel times given only the barrel length, regardless of barrel construction or mounting. Given this barrel time, we can use an internal ballistics program such as Quickload to find powder and charge weight combinations that fulfill the simultaneous requirements of:
Filling the case as full as possible, thereby ensuring the most rapid initial stage possible
Ensure that the powder is all lit before the first pulse passes back over the bullet at the chamber
Meet the overall barrel time requirement based on the length of the barrel
Next Steps
I am planning to instrument a barrel with both strain gauges and acoustic sensors (microphones) in order to get hard confirmation of the above theories. I will update this document as that information becomes available. Any comments, questions, or suggestions can be directed to me at My Email Address .
Good shooting!
More:
http://www.the-long-family.com/OBT_paper.htm
